Running validation tests from a release

[alban]

Document how to do a release including the pre-compiled binaries.

• docs: update link to spec
• docs: move compliance testing to separate doc
• docs: update validation test output
• docs: how to do a release
• docs: how to run tests from a release

Fixes https://github.com/opencontainers/runtime-tools/issues/609

[alban]

@wking thanks for the review! I'll fix that after I finish my current task (test for namespace path).

[dongsupark]

Rebased, and addressed review comments by @wking.